1. Field of the Invention
This invention relates to a method for suppressing the harmful effects of one or more metal contaminants (especially vanadium) on catalysts used in processes that convert higher boiling hydrocarbons to lower boiling fractions. More particularly, this invention relates to the use of a strontium colloid system in catalytic hydrocarbon conversion processes to react with and trap said metal contaminants, thereby resulting in a lower coke make and hydrogen production together with increased activity maintenance of said catalysts.
2. Discussion of Related Art
Today, many modern refineries contain one or more hydrocarbon conversion process (e.g., a fluid catalytic cracking process) in which some of the heavier oils (e.g. gas oils) produced upon fractionation of whole crude oil are converted or "cracked" to lower boiling fractions (e.g. gasoline) in the presence of a suitable catalyst. Usually, oils from light, sweet crudes are preferred feedstocks to most catalytic conversion processes. However, in recent years, the supply of such crudes has diminished, thereby requiring the increasing use of heavier, more sour crudes which contain substantially greater organic metals and metal compounds, such as vanadium and nickel porphyrins. These metal contaminants deposit and accumulate on the catalyst which results in increasing the yield of undesirable hydrogen and coke and decreasing the selectivity of the catalyst to produce desirable liquid products. In addition, vanadium has been found to attack the high activity component(s) of conversion catalysts (e.g. zeolite), thereby causing a decrease in catalyst activity (See for example Ritter et al., "A Look at New FCC Catalysts for Resid", Oil and Gas Journal, July 6, 1981, p. 103). While the mode of vanadium attack is not completely understood, vanadium is believed to migrate through the catalyst particle and accumulate in areas where the high activity component(s) is concentrated. In the case of zeolites, all appear to be susceptible to vanadium attack although the level of susceptibility seems to vary with the type of zeolite and the extent and type of cation exchange.
To counteract the adverse effects of such metal contaminants, various metal additives have been included in hydrocarbon conversion catalysts. Examples of such additives are molecular compounds of antimony (U.S. Pat. Nos. 3,711,422; 4,238,362; 4,279,735; and 4,495,064); bismuth or manganese (U.S. Pat. No. 3,977,963); tin (U.S. Pat. No. 4,101,417 and published European Patent Application No. 0,187,506-A2); barium (U.S. Pat. Nos. 4,377,494 and 4,473,463); calcium (U.S. Pat. Nos. 4,451,355 and 4,520,120); magnesium (U.S. Pat. No. 4,556,478) and lithium (U.S. Pat. No. 4,364,847). Mixtures of various metal additives have also been found to be helpful in reducing the deleterious effects of the metal contaminants. See for example U.S. Pat. No. 4,504,381 (tin and cadmium), U.S. Pat. No. 4,522,704 (cadmium, germanium, indium, tellurium or zinc) and U.S. Pat. No. 4,535,066 (antimony and a composition made by treating a soluble salt of dialkyldithiocarbonate with a hydrolyzable germanium (IV) compound).
Non-colloidal strontium compounds, alone or in combination with other compounds, have also been included in hydrocarbon conversion catalysts and for a variety of reasons. For example, SrO has been used as a component of a catalyst for cracking alkanes (U.S. Pat. No. 4,093,536), as a component of a cracking catalyst support (U.S. Pat. No. 4,382,878) and as a promoter in transition metal oxide Bronsted acid catalysts (U.S. Pat. No. 4,415,480). Non-colloidal strontium compounds (e.g. SrCO.sub.3 and SrSO.sub.4) have been added to improve the attrition resistance of cracking catalysts as well (see U.S. Pat. Nos. 3,030,300 and 3,265,611). Alkaline earth metal compounds have also been used to remove metal contaminants from hydrocarbon feedstocks (see for example U.S. Pat. Nos. 2,902,442 and 3,617,530), while in U.S. Pat. No. 4,396,496 to Scharf et al., certain non-colloidal strontium compounds (namely strontium oxide, strontium alkyl oxide, strontium aryl oxide and strontium sulfate) have been used with antimony to passivate the metal contaminants and to increase the attrition resistance of the catalyst.
Recently, U.S. Pat. No. 4,432,890 to Beck et al., the disclosure of which is incorporated herein by reference, disclosed the use of a metal additive in a zeolite-containing conversion catalyst to immobilize vanadium compounds present in the hydrocarbon feedstock to a conversion process. The additive prevents the vanadium from attacking the zeolitic component and causing the rapid deactivation of said catalyst. In column 7, lines 45-47, patentees list several possible metal additives, including strontium. Patentees also disclose that the metal additive may be added during catalyst manufacture, after manufacture by impregnation or at any point in the conversion process (see column 1, lines 16-20). When added to the conversion process, "the metal additives are preferably organo-metallic compounds of these metals soluble in the hydrocarbon feed or in a hydrocarbon solvent miscible with the feed." (see column 10, lines 33-36).
Another patent to Beck et al. (U.S. Pat. No. 4,549,958), the disclosure of which is incorporated herein by reference, is related to Beck et al.'s U.S. Pat. No. 4,432,890 patent in that similar metal additives (including strontium) are used to immobilize vanadium. However, in the '958 patent, the immobilization occurs on sorbent particles (preferably dehydrated kaolin clay) treated with the metal additives which circulate through the conversion process along with the zeolite-containing conversion catalyst. The metal contaminants are deposited on the sorbent particles and immobilized thereon upon contact with the metal additive. Suitable additives include the metals, their oxides and salts, and the organo-metallic compounds of the metals listed at column 8, lines 11-14.
Colloids of antimony-containing compounds have also been used to restore the activity of molecular sieve cracking catalysts which have been contaminated with metals such as vanadium and nickel (see, for example, U.S. Pat. No. 4,483,765 to Payne and published European Patent Application No. 0,130,543-A2 to Kaplan). The colloid is typically introduced with the hydrocarbon feedstock, or a portion thereof, to the cracking process. However, colloids containing strontium, while known in the art (see U.S. Pat. No. 3,372,116 to Meinhardt), have not been used in a hydrocarbon conversion process, and specifically not in a catalytic cracking process.
More recently, U.S. application Ser. No. 743,593 to Kugler disclosed a fluid catalytic cracking catalyst containing a zeolite and a particulate, substantially water insoluble non-colloidal strontium compound in an amount effective to mitigate the effects of metal contaminants contained in the hydrocarbon feedstock to the cracking process. However, neither this reference nor any of the other references discussed above, alone or in combination, teach or suggest using a strontium colloid system in a hydrocarbon conversion process for any purpose, much less for trapping and immobilizing metal contaminants present in the feedstock to said process.